1. Field of the Invention
This invention relates to an active matrix display device, specifically to an active matrix display device with an improved display quality.
2. Description of the Related Art
A flat panel display device including a reflection type active matrix liquid crystal display device (referred to as LCD hereinafter) can be thin, small and light, and it operates with low-power consumption. The LCD has been used as a display part in various devices such as mobile information device. The LCD, whose pixel has a switching element and a thin film transistor, is known as an active matrix type. The panel of the active matrix display device is highly reliable for maintaining displayed contents of the pixels, which provides the active matrix display device with high display quality.
FIG. 7 shows an equivalent circuit of a pixel in the active matrix LCD. Each pixel has a thin film transistor (TFT) 11 connected to a gate line and a data line. When the TFT is turned on by a selection signal outputted to the gate line, the data corresponding to the display content is supplied to a liquid crystal capacitance 12 (Clc) from the data line through the TFT. It is necessary to accurately keep the display data from the time when the TFT is first selected for writing to the time when the TFT is selected again in the next sequence. Therefore, a storage capacitance 13 (Csc) is connected to the TFT in series with the liquid crystal capacitance Clc.
FIG. 8 is a plan view showing the configuration of the pixel portion on a TFT forming substrate (a first substrate 100 in FIG. 9) of the conventional LCD. FIG. 9 is a cross-sectional view of the LCD configuration along with the X-X cross sectional line in FIG. 8. The LCD has a first substrate and a second substrate with the liquid crystal between them. In the active matrix LCD, the TFTs 11 and pixel electrodes 74 are arranged in a matrix configuration on the first substrate 100. A common electrode 56, to which a common voltage Vcom is supplied, and a color filter 54 are disposed on the second substrate 500, which is disposed facing to the first substrate 100. The voltage applied between the pixel electrode 74 and the common electrode 56, which are facing each other with the liquid crystal between them, drives the liquid crystal capacitance Clc.
The TFT disposed for each of the pixels on the first substrate 100 side is a top-gate type TFT, whose gate electrode is located above an active layer 64, as seen from FIG. 9. The active layer 64 of the TFT is patterned on the substrate 100 as shown in FIG. 8. A gate insulating layer 66 is disposed covering over the active layer 64, and the gate line, which also functions as a gate electrode, is disposed on the gate insulating layer 66. The part of the active layer 64 facing against the gate electrode is a channel region. A drain region 64d and a source region 64s with an impurity doped are formed at the corresponding sides of the channel region.
The drain region 64d of the active layer 64 is connected to the data line, which functions also as a drain electrode 70, through a contact hole formed in an interlayer insulating layer 68 covering the gate electrode.
Also, a flattening insulating layer 72 is disposed covering the data line and the drain line 70. The source region 64s of the active layer 64 is connected to pixel electrode 74 made of ITO (Indium Tin Oxide) on the flattening insulating layer 72 through a contact hole.
The source region 64s of the active layer 64 functions also as a first electrode 80 of the storage capacitance Csc disposed for each of the pixels and extends further, as shown in FIG. 8, from the contact region of the pixel electrode 74. A second electrode 84 of the storage capacitance element Csc is formed simultaneously with and in the same layer as the gate electrode as seen from FIG. 9, but it is formed in a region away from the gate electrode, keeping a certain distance between them. The gate insulating layer 66 also works as a dielectric between the first electrode 80 and the second electrode 84. The second electrode 84 of the storage capacitance element Csc, are not independently disposed for each of the pixel as seen from FIG. 8. But it is disposed in the pixel region along the row direction of the matrix in the same manner as the gate line 60. A predetermined storage capacitance voltage Vsc is applied to the second electrode 84.
The storage capacitance element Csc disposed for each of the pixels maintains the electric charge corresponding to the display contents, which should be applied to the liquid crystal Clc, when the TFT is not selected. Therefore, the voltage change of the pixel electrode 74 can be maintained, enabling the display contents to be kept unchanged during one sequence.
The gate line 60 and the second electrode 84 (a storage capacitance line) for forming the storage capacitance element Csc are disposed in parallel. The location of these two lines with respect to the location of the pixel electrode 74 is shown in FIG. 10. The gate line 60 and the second electrode 84 (a storage capacitance line) are disposed in the layer under the pixel electrodes 74, adjacent to each other. The portions of the lines between the pixels shown as shaded areas in the figure are not covered by either of the pixel electrodes 74.
The liquid crystal corresponding to the portions of the lines shown as shaded areas occasionally shows white light. This is often observed when the adjacent pixels appear as black elements. When white light is observed, the display quality is deteriorated.
The gate line 60 and the second electrode 84 (the storage capacitance line) are usually made of reflecting material such as aluminum, molybdenum and chrome, which reflect light. Therefore, because the pixel electrodes 74, which control the reflection of the light, do not exist at the upper layer of the lines (shaded in the figure) in the conventional device, the alignment of the liquid crystal corresponding to this portion can not be controlled. This leads to a frequent white light observation. Although this problem can be solved by disposing a black matrix (BM), the aperture ratio would be reduced.
Therefore, this invention is directed to prevent the abovementioned problem and to offer the active matrix display device of high quality display.